During displaying with a liquid crystal display, an image is displayed by outputting a signal from a drive circuit inside the liquid crystal display and scanning pixel units of the liquid crystal display line by line. In a liquid crystal display, scanning for an image line by line is mainly realized by a shift register. FIG. 1 shows a structural diagram of a shift register in the prior art. As shown in FIG. 1, the shift register comprises shift register units connected in multiple stages, a low voltage supply line connected with a low level input terminal VSS, a first clock signal supply line connected with a first clock signal input terminal CLK and a second clock signal supply line connected with a second clock signal input terminal CLKB, wherein, a signal from an output terminal OUTPUT of a shift register unit in nth stage is provided to a reset terminal RESET of a shift register unit in (n−1)th stage as a reset signal, and is provided to an input terminal INPUT of a shift register unit in (n+1)th stage as an input signal.
However, only a method of scanning in a single direction from top to bottom can be realized by such a shift register, so that a liquid crystal display provided with such a shift register has a single display mode, display flexibility thereof is poor, and different requirements for display cannot be satisfied.
Therefore, how to realize a bidirectional scanning for a liquid crystal display becomes a technical problem to be solved in the art.